In recent years the theory that measurement of the potential level of the electromagnetic field of a living organism can be used as an accurate screening and diagnostic tool is gaining greater acceptance. Many methods and devices have been developed in an attempt to implement this theory. For example, U.S. Pat. No. 4,328,809 to B. H. Hirschowitz et al. deals with a device and method for detecting the potential level of the electromagnetic field present between a reference point and a test point on a living organism. Here, a reference electrode and a test electrode provide DC signals indicative of the potential level of the electromagnetic field measured between the reference point and the test point. These signals are provided to an analog-to-digital converter which generates a digital signal as a function thereof, and a processor provides an output signal indicative of a parameter or parameters of the living organism as a function of this digital signal.
Similar biopotential measuring devices are shown by U.S. Pat. Nos. 4,407,300 to Davis, and 4,557,271 and 4,557,273 to Stroller et al. Davis, in particular, discloses the diagnosis of cancer by measuring the electromotive forces generated between two electrodes applied to a subject.
Often, the measurement of biopotentials has been accomplished using an electrode array, with some type of multiplexing system to switch between electrodes in the array. The aforementioned Hirschowitz et al. patent contemplates the use of a plurality of test electrodes, while U.S. Pat. Nos. 4,416,288 to Freeman and 4,486,835 to Bai disclose the use of measuring electrode arrays.
Unfortunately, previous methods for employing biopotentials measured at the surface of a living organism as a diagnostic tool, while basically valid, are predicated upon an overly simplistic hypothesis which does not provide an effective diagnosis for many disease states. Prior methods and devices which implement them operate on the basis that a disease state is indicated by a negative polarity which occurs relative to a reference voltage obtained from another site on the body of a patient, while normal or non-malignant states, in the case of cancer, are indicated by a positive polarity. Based upon this hypothesis, it follows that the detection and diagnosis of disease states can be accomplished by using one measuring electrode situated externally on or near the disease site to provide a measurement of the polarity of the signal received from the site relative to that from the reference site. Where multiple measuring electrodes have been used, their outputs have merely been summed and averaged to obtain one average signal from which a polarity determination is made. This approach can be subject to major deficiencies which lead to diagnostic inaccuracy, particularly where only surface measurements are taken.
First, the polarity of diseased tissue underlying a recording electrode has been found to change over time. This fact results in a potential change which confounds reliable diagnosis when only one external recording electrode is used. Additionally, the polarity of tissue as measured by skin surface recording is dependent not only upon the placement of the recording electrode, but also upon the placement of the reference electrode. Therefore, a measured negative polarity is not necessarily indicative of diseases such as cancer, since polarity at the disease site depends in part on the placement of the reference electrode.
As disease states such as cancer progress, they produce local effects which include changes in vascularization, water content, and cell division rate. These effects alter ionic concentrations which can be measured at the skin surface and within the neoplastic tissues. Other local effects, such as distortions in biologically closed electrical circuits, may occur. A key point to recognize is that these effects do not occur uniformly around the disease site. For example, as a tumor grows and differentiates, it may show wide variations in its vascularity, water content and cell division rate, depending on whether examination occurs at the core of the tumor (which may be necrotic) or at the margins of the tumor (which may contain the most metabolically active cells). The tumor may not respond significantly to growth factors, while the growth factors and the enzymes produced may significantly affect the normal cells surrounding the tumor. Once this fact is recognized, it follows that important electrical indications of disease are going to be seen in the relative voltages recorded from a number of sites at and near a diseased area, and not, as previously assumed, on the direction (positive vs. negative) of polarity.
The accurate measurement of DC biopotentials for sensing or screening for disease, injury or bodily functions is very difficult to accomplish, for the DC potentials to be sensed are of a very low amplitude. Due to factors such as the low DC potentials involved and the innate complexity of biological systems, the collected data signals tend to include a substantial volume of noise which makes accurate analysis difficult. Also, biological systems are notorious for their complexity, nonlinearity and nonpredictability, and wide variations from the norm are not uncommon. For example, DC biopotential signals tend to drift over time, so that if signals are not sensed and analyzed with some rapidity, signal errors due to drift occur.
For the accurate measurement of DC biopotentials for disease diagnosis and screening, electrode and electrode circuit characteristics and electrode placement become important. Factors such as small DC offset potentials in the low millivolt range, which may have little effect on an AC biopotential measurement, such as ECG measurement, can destroy the accuracy of a DC biopotential measurement. For screening applications where many sensing electrodes are used, it is often critical for electrode characteristics to be uniform, for accurate electrode spacing to be maintained and for DC offsets to be substantially eliminated.
Many DC biopotential sensing electrodes are packaged in a pre-gelled state wherein an electrolytic paste or gel is packaged as part of the electrode. The gel may be located in a central gel reservoir consisting of a molded cup, or it may be contained in a dye-cut hole in a foam which encapsulates a gel saturated open cell compressible foam column. In most instances, the pre-gelled electrodes are sold ready for use with an electrically conductive material such as metal or a metal chloride in contact with the electrolyte gel.
A pre-gelled electrode system is generally not a battery by itself, but forms a part of a battery-system consisting of two or more electrodes placed on the body. In such a system, a complex battery is formed consisting of many interactive components including the electrode material (frequently silver/silver chloride), the electrode gel, internal body chemistry and external skin conditions, skin preparation, temperature, air condition and chemistry, etc. Obviously, some of these factors are not subject to control, but in order to get the best data possible, especially in instances where DC biopotentials are of interest, artifacts, such as DC offsets, should be reduced to the lowest level. Most pre-gelled electrodes when introduced in the battery system outlined above contribute some unwanted DC voltage (polarization effect) to biopotential measurements. It is important to lower the possibility of such DC artifacts occurring to a degree sufficient to preclude them from having a substantial adverse effect on biopotential measurements.
The design and performance characteristics for an effective DC biopotential electrode are different from those of electrodes designed for measuring alternating current (AC) signals such as those used with electrocardiology (ECG) and electroencephalography (EEG). For example, U.S. national standards for single use ECG electrodes allow the DC offset of an electrode pair (i.e., fie spurious DC current generated by electrochemical interactions between electrode components) to be as high as 100 millivolts (ANSI/AAMI standard). Since effective use of DC signals for cancer diagnosis requires discrimination at the one millivolt level, standards for ECG electrodes are grossly excessive. ECG electrodes are intended for AC measurements which are not significantly affected by DC offset voltages in the electrode to the degree that DC biopotential measurements are adversely affected by such offset voltages. The traditional view taken in the manufacture of ECG pregelled electrodes is that to reduce DC offset, one must sacrifice AC impedance, and since a low AC impedance is most important in an ECG pregelled electrode, the DC offset voltage is tolerated. However, for highly accurate DC biopotential measurements, both the DC offset potential and the AC impedance for the electrode must be low.
When DC biopotential measurements are taken from the skin of a subject with conventional EGG electrodes, sweat gland activity creates noise which tends to mask the sensed DC signal, and the provision of an electrode structure to minimize this noise is essential.
If a pre-gelled electrode array is to be used effectively for disease detection, such as breast cancer screening, the array will require a relatively large number of spaced electrodes to cover substantially the entire surface of the breast. Not only must each of these electrodes be free from error causing offset potentials before use, but the electrodes must maintain contact with the curved surface of the breast without movement during the screening procedure and must maintain a predetermined array formation with specified electrode spacing. Consistent location and orientation of the electrical channels connected to the respective electrodes must also be maintained to prevent incorrect connection to the electrodes and to maintain positive contact between the electrodes and the electrical channel leads therefor.
In the past, these objectives have not been met with an apparatus which could be rapidly applied and worn comfortably by a patient, and which also produces reliable DC measurements.
The key to effective measurement and analysis of direct current skin potentials is absolute maintenance of signal integrity from the skin surface to the signal processing components of the measuring unit. This is especially critical due to the inherent low amplitude of biologic DC potentials. At any point in the electronic path from the skin sensing electrode to the measuring unit, potential exists for noise to intrude upon signal, thereby degrading diagnostically useful information.